1. Field of the Invention
This invention relates to the production of novel metal modified novolak resins containing salicylic acid. These metal modified novolak resins can be coated on a paper substrate to produce a pressure-sensitive paper having improved print character and stability of print on ageing.
2. Prior Art
Pressure-sensitive carbonless copy paper, briefly stated, is a standard type of paper wherein during manufacture the backside of a paper substrate is coated with what is referred to as CB coating, the CB coating containing one or more color precursors generally in capsular, and more preferably microcapsular, form. At the same time, the front side of the paper substrate is coated during manufacture with what is referred to as a CF coating which contains one or more color developers. Both the color precursor and the color developer remain in the coating compositions on the respective back and front surfaces of the paper in colorless form. This is true until the CB and CF coatings are brought into abutting relationship and sufficient pressure, as by a typewriter, is applied to rupture the CB coating to release the color precursor. At this time the color precursor transfers to the CF coating and reacts with the color developer therein to form an image. Patents relating to carbonless paper products are:
U.S. Pat. No. 2,712,507 (1955) to Green. PA1 U.S. Pat. No. 2,730,456 (1956) to Green et al. PA1 U.S. Pat. No. 3,455,721 (1969) to Phillips et al. PA1 U.S. Pat. No. 3,466,184 (1969) to Bowler et al. PA1 U.S. Pat. No. 3,672,935 (1972) to Miller et al.
Another pressure-sensitive product which is in an advanced stage of development and commercialization at this time and which is available in some business sectors is referred to as self-contained paper. Very generally stated, self-contained paper refers to an imaging system wherein only one side of the paper needs to be coated and the one coating contains both the color precursor, generally in encapsulated form, and the color developer. Thus, when pressure is applied, again as by a typewriter or other writing instrument, the color precursor capsule is ruptured and reacts with the surrounding color developer to form an image. Both the carbonless paper image transfer system and the self-contained system have been the subject of a great deal of patent activity. A typical autogenous record material system, earlier sometimes referred to as "self-contained" because all elements for making a mark are in a single sheet, is disclosed in U.S. Pat. No. 2,730,457 (1956) to Green.
U.S. Pat. Nos. 3,455,721, 3,466,184 and 3,672,935, all mentioned above, disclose the use of phenol-aldehyde (novolak) resins as color developers (electron acceptors) in the production of pressure-sensitive CF record sheets. These novolak resins can be applied to paper either as a dispersion of ground resin or a solution of resin in an organic solvent to form the record sheet. Such record sheets can be used in conjunction with transfer sheets containing colorless color precursors of the electron donor type such as Crystal Violet Lactone.
The use of metal compounds as a means of improving the color-forming reactions of novolak resins with electron donor color precursors is known. Acidic water soluble metal salts incorporated in aqueous coatings of ground novolak resins are disclosed in U.S. Pat. No. 3,516,845 (1970) to Brockett while U.S. Pat. No. 3,723,156 (1973) to Brockett et al. discloses a further improvement through a similar use of oil soluble metal salts. U.S. Pat. Nos. 3,732,120 (1973) to Brockett et al. and 3,737,410 (1973) to Mueller disclose the interaction of a metal compound such as zinc hydroxy benzoate, zinc acetylacetonate and zinc dibenzoate with a para-substituted novolak resin by melting the resin and metal compound together to give a color developer which shows increased color intensity and resistance to fading. U.S. Pat. No. 4,025,490 (1977) to Weaver discloses metal-modified para-substituted novolak resins prepared by melting together a novolak resin and a metal salt of an alkanoic acid in the presence of an ammonium compound. These resins result in significant improvements, when used in a manifolding system, in rate of color image development, stability of image on ageing (fade resistance) and storage stability prior to imaging (decline).
U.S. Pat. No. 3,952,132 (1976) to Kato et al. discloses the preparation of a reacted compound between an alkali metal salt of a phenol resin, including novolaks, and a water soluble metallic compound. Recording sheets containing this reacted compound have improved developing ability and resistance to light irradiation. U.S. Pat. No. 3,772,052 (1973) to Kimura et al. discloses preparation of a color developer metal compound of a polymer which is the reaction product of an aromatic carboxylic acid containing at least one hydroxyl group, such as salicylic acid, with an aldehyde or acetylene. The metal compound of the polymer improved color developing ability and light resistance over the carboxylic acid or the metal compound.
U.S. Pat. Nos. 3,874,895 (1975) to Hayashi et al. and 3,924,027 (1975) to Saito et al. disclose the preparation of color developers which are physical mixtures of: (1) an organic polymer, such as phenol-formaldehyde condensates or substituted phenol-formaldehyde condensates, and (2) a carboxylic acid which may be salicylic acid or a metal salt thereof. Improved resistance to heat, light and ageing and improved color developing ability are among the advantages of these color developers.
Most of the above color developers which contain an aromatic carboxylic acid, such as salicylic and, or metal salts thereof have the disadvantage of being insoluble or only slightly soluble in organic solvents. Thus, these color developers are not adaptable to application in solvent solutions to a paper substrate in the manner disclosed in U.S. Pat. No. 3,466,184.
I have found that novel color developers containing salicylic acid in reacted form with para-substituted phenols and formaldehyde can be prepared which are soluble in most organic solvents and can be applied to paper in solvent solution to give a pressure-sensitive record sheet of qualities superior to those found in the prior art. These novel metal modified novolak resins can be prepared by reacting, in the presence of acid catalyst, a mixture of a para-substituted phenol, salicylic acid and formaldehyde and melting the reacted mixture with a metal containing salt. The above process has been used for preparing novolak resins formed by the reaction of a para-substituted phenol and formaldehyde, but have not been used for the novolak resins of the instant invention in which a part of the para-substituted phenol has been replaced by salicylic acid.
Record sheets prepared using the novel metal modified novolak resins of this invention have shown improved intensity and rate of color image development and improved stability of the image on ageing (fade resistance) and storage stability prior to imaging (decline) over record sheets prepared from the prior art metal modified novolak resins.